In recent years, digitalization of media, in particular, bound media, such as books and magazines, has been demanded. Methods of digitalizing bound media include: a method of separating each sheet from a bound medium and reading an image of each sheet by use of a normal image reading apparatus; and a method of reading images of a bound medium by turning over pages of the bound medium without unbinding the bound medium. As to the latter method, an image reading apparatus has been proposed, which reads, at once, a bound medium, which has been opened, and for which pages of the bound medium are turned over manually (for example, in Japanese Laid-open Patent Publication No. 2005-086443, Japanese Patent No. 2914260, Japanese Laid-open Patent Publication No. 2007-082005, and Japanese Laid-open Patent Publication No. 2006-166378).
Further, depending on bound media, placing a bound medium, such as, for example, an old hook, on a horizontal placement table and reading the bound medium may cause the hound medium to be opened too much and may damage the bound medium. Therefore, an image reading apparatus has been proposed, which enables a placement table to be, not only in a horizontal state, but also in an inclined state, where a bound medium is prevented. from being opened too much (for example, in Bookeye4 [online], Image Access, 2013 [retrieved on Aug. 9, 2014], retrieved from the Internet: <URL:http://www.imageaccess.com/bookeye4.shtml>).
When a bound medium is read from a vertical direction with respect to a placement table, change in distance in the vertical direction between the bound medium and an image reading unit that performs reading in an image reading apparatus is small in a horizontal state, but in an inclined state, the change is large. Therefore, when the image reading unit reads the bound medium by the same operation in both the horizontal state and the inclined state of the placement table, the same image quality is difficult to be obtained.